It is well-known in the field of printing to print indicia onto moving documents by use of a print head having printing elements with fixed spacing. These printing elements are typically ink jets. Printing resolution is generally limited to the fixed printing element spacing of the particular print head. The printing resolution is defined in pixels or dots per unit length. Conventionally, this is expressed as dots per inch (dpi).
In a conventional printing apparatus, a print head is oriented generally orthogonally to the process direction of the paper path. Increases in print resolution are limited by the ability to more closely space the printing elements of the print head. To overcome the limitations on the fixed spacing of the printing elements, a prior printing apparatus has the print head oriented at an angle to the process direction of the paper path. The angled orientation of the print head operatively decreases the relative distance between adjacent printing elements of the print head in the direction orthogonal to the process direction. An example of a printing apparatus having an angled print head is disclosed in U.S. patent application Ser. No. 08/552,789 entitled "A Printer Assembly", filed Nov. 3, 1995, which is incorporated by reference herein.
Conventional printing apparatus having the print head generally perpendicular to the process direction of the document are operated to print generally simultaneously along the entire length of the print head. Therefore, each line of pixels orthogonal to the process direction, or scan line, has the pixels printed simultaneously.
Conventionally, the print head is operated whereby data is "loaded" into the print head. In a monochrome print head, the data is for a particular printing element to print a pixel, the pixel being an ink dot or a space. The print head is then at the appropriate time triggered to simultaneously "fire" the printing elements, whereby all of the printing elements are simultaneously triggered to perform the particular data loaded therein. The desired image is thereby generated by a combination of ink dots and spaces printed by the printing elements.
In an angled print head arrangement, each pixel of a particular scan line is printed sequentially instead of simultaneously. The first pixel of a scan line on a document is printed by the first printing element of the angled print head. A second pixel is printed adjacent the first pixel by the second printing element as the document is moved in a process direction. The document then continues to move in the process direction whereby the third pixel can be printed adjacent the second pixel by the third printing element, etc.
In an actual printer operating environment, the angled print head functions to simultaneously print multiple scan lines, the number of scan lines being simultaneously printed generally equal to the number of printing elements of the print head. As a document moves in the process direction, the scan line furthest in the process direction is being printed by the last printing element of the print head, while simultaneously the first printing element of the print head is printing the newest scan line. Therefore, in the process direction, the first printing element initially prints the first pixel of a new scan line. Simultaneously, the final printing element completes printing the final pixel on the final scan line that has been previously sequentially printed on by each previous printing elements of the print head.
Operationally, in a simplified example having four printing elements on an angled print head, the printing procedure is as follows for printing four scan lines. The first printing element prints the first pixel of the first scan line. The document is then moved in the process direction. The second printing element at a later time prints a second pixel adjacent the first pixel on the first scan line. Simultaneous with the printing of the second pixel, the first printing element prints the first pixel of the second scan line. In the next subsequent step, the document is again moved in the process direction, the third printing element prints the third pixel of the first scan line simultaneously with the second printing element printing the second pixel of the second scan line and the first printing element printing the first pixel of the third scan line. The printing process is continued until the entire image is produced on the document.
In order to ensure that each scan line is straight, it is preferable that the distance in the process direction between adjacent printing elements be an integer number of scan lines. Integer spacing of adjacent printing elements in the process direction assure that each subsequent pixel printed on a particular scan line is adjacent to the previously printed pixel. The integer spacing is preferable due to the firing of all of the printing elements simultaneously to print multiple scan lines. Non-integer spacing between the printing elements in the process direction sacrifices line straightness in order to allow a particular resolution or dpi perpendicular to the process direction. The displacement of a particular pixel from a line through the first pixel of a scan line and orthogonal to the process direction is referred to as pixel placement error.